Display driver circuits

ABSTRACT

This disclosure generally relates to display driver circuits for electro-optic displays, and more particularly relates to circuits and methods for driving active matrix organic light emitting diode displays with greater efficiency. A display driver for an electroluminescent display, the display including a plurality of electroluminescent display elements each associated with a display element driver circuit, each display element driver circuit including a drive transistor having a control connection for driving the associated display element in accordance with a voltage on the control connection, the display driver including at least one display element brightness controller to provide an output to drive a control connection to control the electroluminescent output from a display element; a voltage sensor to sense the voltage on a control connection; and a power controller for controlling an adjustable power supply for providing an adjustable voltage to the electroluminescent display to power said drive transistors for driving said display elements, the power controller being configured to provide a control signal to adjust said power supply voltage in response to said sensed voltage.

This is the U.S. national phase of International Application No.PCT/GB03/02529 filed Jun. 11, 2003, the entire disclosure of which isincorporated herein by reference.

This invention generally relates to display driver circuits forelectro-optic displays, and more particularly relates to circuits andmethods for driving active matrix organic light emitting diode displayswith greater efficiency.

Organic light emitting diodes (OLEDs) comprise a particularlyadvantageous form of electro-optic display. They are bright, colorfulfast-switching, provide a wide viewing angle and are easy and cheap tofabricate on a variety of substrates. Organic LEDs may be fabricatedusing either polymers or small molecules in a range of colours (or inmulti-coloured displays), depending upon the materials used. Examples ofpolymer-based organic LEDs are described in WO 90/13148, WO 95/06400 andWO 99/48160; examples of so called small molecule based devices aredescribed in U.S. Pat. No. 4,539,507.

A basic structure 100 of a typical organic LED is shown in FIG. 1 a. Aglass or plastic substrate 102 supports a transparent anode layer 104comprising, for example, indium tin oxide (ITO) on which is deposited ahole transport layer 106, an electroluminescent layer 108, and a cathode110. The electroluminescent layer 108 may comprise, for example, a PPV(poly(p-phenylenevinylene)) and the hole transport layer 106, whichhelps match the hole energy levels of the anode layer 104 andelectroluminescent layer 108, may comprise, for example, PEDOT:PSS(polystyrene-sulphonate-doped polyethylene-dioxythiophene). Cathodelayer 110 typically comprises a low work function metal such as calciumand may include an additional layer immediately adjacentelectroluminescent layer 108, such as a layer of aluminium, for improvedelectron energy level matching. Contact wires 114 and 116 to the anodethe cathode respectively provide a connection to a power source 118. Thesame basic structure may also be employed for small molecule devices.

In the example shown in FIG. 1 a light 120 is emitted throughtransparent anode 104 and substrate 102 and such devices are referred toas “bottom emitters”. Devices which emit through the cathode may also beconstructed, for example by keeping the thickness of cathode layer 110less than around 50-100 nm so that the cathode is substantiallytransparent.

Organic LEDs may be deposited on a substrate in a matrix of pixels toform a single or multi-colour pixellated display. A multicoloureddisplay may be constructed using groups of red, green, and blue emittingpixels. In such displays the individual elements are generally addressedby activating row (or column) lines to select the pixels, and rows (orcolumns) of pixels are written to, to create a display. It will beappreciated that with such an arrangement it is desirable to have amemory element associated with each pixel so that the data written to apixel is retained whilst other pixels are addressed. Generally this isachieved by a storage capacitor which stores a voltage set on a gate ofa driver transistor. Such devices are referred to as active matrixdisplays and examples of polymer and small-molecule active matrixdisplay drivers can be found in WO 99/42983 and EP 0,717,446Arespectively.

FIG. 1 b shows such a typical OLED driver circuit 150. A circuit 150 isprovided for each pixel of the display and ground 152, V_(ss) 154, rowselect 164 and column data 166 busbars are provided interconnecting thepixels. Thus each pixel has a power and ground connection and each rowof pixels has a common row select line 164 and each column of pixels hasa common data line 166.

Each pixel has an organic LED 156 connected in series with a drivertransistor 158 between ground and power lines 152 and 154. A gateconnection 159 of driver transistor 158 is coupled to a storagecapacitor 160 and a control transistor 162 couples gate 159 to columndata line 166 under control of row select line 164. Transistor 162 is afield effect transistor (FET) switch which connects column data line 166to gate 159 and capacitor 160 when row select line 164 is activate& Thuswhen switch 162 is on a voltage on column data line 166 can be stored ona capacitor 160. This voltage is retained on the capacitor for at leastthe frame refresh period because of the relatively high impedances ofthe gate connection to driver transistor 158 and of switch transistor162 in its “off” state.

Driver transistor 158 is typically an FET transistor and passes a (drainsource) current which is dependent upon the transistor's gate voltageless a threshold voltage. Thus the voltage at gate node 159 controls thecurrent through OLED 156 and hence the brightness of the OLED.

The standard voltage-controlled circuit of FIG. 1 b suffers from anumber of drawbacks. The main problems arise because the brightness ofOLED 156 is dependent upon the characteristics of the OLED and of thetransistor 158 which is driving it In general, these vary across thearea of a display and with time, temperature, and age. This makes itdifficult to predict in practice how bright a pixel will appear whendriven by a given voltage on column data line 166. In a colour displaythe accuracy of colour representations may also be affected.

FIG. 2 a shows a current-controlled pixel driver circuit 200 whichaddresses these problems. In this circuit the current through an OLED216 is set by setting a drain source current for OLED driver transistor212 using a reference current sink 224 and memorising the drivertransistor gate voltage required for this drain-source current. Thus thebrightness of OLED 216 is determined by the current, I_(col′), flowinginto reference current sink 224, which is preferably adjustable and setas desired for the pixel being addressed. It will be appreciated thatone current sink 224 is provided for each column data line 210 ratherthan for each pixel.

In more detail, power 202, 204, column data 210, and row select 206lines are provided as described with reference to the voltage-controlledpixel driver of FIG. 1 b. In addition an inverted row select line 208 isalso provided, the inverted row select line being high when row selectline 206 is low and vice versa A driver transistor 212 has a storagecapacitor 218 coupled to its gate connection to store agate voltage fordriving the transistor to pass a desired drain-source current Drivetransistor 212 and OLED 216 are connected in series between a power 202and ground 204 lines and, in addition, a further switching transistor214 is connected between drive transistor 212 and OLED 216, transistor214 having a gate connection coupled to inverted row select line 208.Two further switching transistors 220, 222 are controlled bynon-inverted row select line 206.

In the embodiment of the current-controlled pixel driver circuit 200illustrated in FIG. 2 a all the transistors are PMOS, which ispreferable because of their greater stability and better resistance tohot electron effects. However NMOS transistors could also be used

In the circuit of FIG. 2 a the source connections of the transistors aretowards GND and for present generation OLED devices V_(ss) is typicallyaround −6 volts. When the row is active the row select line 206 is thusdriven at a more negative voltage, up to approximately −20 volts andinverted row select line 208 is driven at 0 volts.

When row select is active transistors 220 and 222 are turned on andtransistor 214 is turned off. Once the circuit has reached a steadystate reference current I_(col′) into current sink 224 flows throughtransistor 222 and transistor 212 (the gate of 212 presenting a highimpedance). Thus the drain-source current of transistor 212 issubstantially equal to the reference current set by current sink 224 andthe gate voltage required for this drain-source current is stored oncapacitor 218. Then, when row select becomes inactive, transistors 220and 222 are turned off and transistor 214 is turned on so that this samecurrent now flows through transistor 212, transistor 214, and OLED 216.Thus the current through OLED is controlled to be substantially the sameas that-set by reference current sink 224.

Before this steady state is reached the voltage on capacitor 218 willgenerally be different from the required voltage and thus transistor 212will not pass a drain source current equal to the current, I_(col), setby reference sink 224. When such a mismatch exists a current equal tothe difference between the reference current and the drain-sourcecurrent of transistor 212 flows onto or off capacitor 218 throughtransistor 220 to thereby change the gate voltage of transistor 212. Thegate voltage changes until the drain-source current of transistor 212equals the reference current set by sink 224, when the mismatch iseliminated and no current flows through transistor 220.

In the circuit of FIG. 2 a the maximum (most negative) gate voltagedrive is V_(ss). To permit a greater (more negative) drive voltagereference sink 224 may be connected to a drive voltage V_(drive) morenegative than V_(ss).

The circuit of FIG. 2 a solves some of the problems associated with thevoltage-controlled circuit of FIG. 1 b as the current through OLED 216can be set irrespective of variations in the characteristics of pixeldriver transistor 212. However it is still prone to variations in thecharacteristics of OLED 216 between pixels, between active matrixdisplay devices, and with temperature and time.

For this reason optical feedback may be-employed to control the OLEDcurrent, as described in WO 01/20591, EP 0,923,067A, EP 1,096,466A, andJP 5-035,207, which all employ the same basic technique. FIG. 2 b, whichis taken from WO 01/20591, illustrates the technique, which is toconnect a photodiode across the storage capacitor.

FIG. 2 b shows a voltage-controlled pixel driver circuit 250 withoptical feedback 252. The main components of the driver circuit 250 ofFIG. 2 b correspond to those of circuit 150 of FIG. 1 b, that is, anOLED 254 in series with a driver transistor 256 having a storagecapacitor 258 coupled to its gate connection. As illustrated, the pixeldriver circuit has connections 251 and 253 to, respectively, apositive-supply V_(D) and to Ground and driver transistor is an NMOStransistor. The skilled person will appreciate that the circuit couldalso employ a PMOS driver transistor and a negative supply.

A switch transistor 260 is controlled by a row conductor 262 and, whenswitched on, allows a voltage on capacitor 258 to be set by applying avoltage signal to column conductor 264 or a given charge to be injectedinto the capacitor. Additionally, however, a photodiode 266 is connectedacross storage capacitor 258 so that it is reverse biased. Thusphotodiode 266 is essentially non-conducting in the dark and exhibits asmall reverse conductance depending upon the degree of illumination. Thephysical structure of the pixel is arranged so that OLED 254 illuminatesphotodiode 266, thus providing an optical feedback path 252.

The photocurrent through photodiode 266 is approximately linearlyproportional to the instantaneous light output level from OLED 254. Thusthe charge stored on capacitor 258, and hence the voltage across thecapacitor and the brightness of OLED 254, decays approximatelyexponentially over time. The integrated light output from OLED 254, thatis the total number of photons emitted and hence the perceivedbrightness of the OLED pixel, is thus approximately determined by theinitial charge stored on capacitor 258.

Improvements to the circuit of FIG. 2 b, in which every pixel of thedisplay needs refreshing every frame, are described in the applicant'sco-pending UK patent applications 0126120.5 and 0126122.1, both filed on31 Oct. 2001.

FIG. 3 a shows a current-controlled organic LED active matrix pixeldriver circuit 300 with optical feedback according to as described inpatent application number 0126120.5. In the circuit of FIG. 3a, and inthe circuits described later, the transistors of the active matrixpixels are preferably PMOS.

In an active matrix display typically each pixel is provided with such apixel driver circuit. Further driver circuitry (not shown in FIG. 3 a)is provided to address the pixels row-by-row, to set each row at thedesired brightness. To power and control the pixel driver circuitry andOLED display element such an active matrix display is provided with agrid of electrodes including, as shown in FIG. 3 a, a ground (GND) line302, a power or V_(ss) line 304, a row select line 306 and a column dataline 308. Each column data line is connected to a programmable constantcurrent reference source (or sink) 324. This is not part of the drivercircuitry provided for each pixel but instead comprises part of thedisplay driver circuitry provided for each column. Reference currentgenerator 324 is programmable so that it can be adjusted to a desiredlevel to set a pixel brightness, as described in more detail below.

The pixel driver circuit 300 comprises a driver transistor 310 connectedin series with an organic LED display element 312 between the GND 302and V_(ss) 304 lines. A storage capacitor 314, which may be integratedwith the gate of transistor 310, stores a charge corresponding to amemorised gate voltage to control the drive-current through OLE element312. Control circuitry for the driver comprises two switchingtransistors 320, 322 with a common gate connection coupled to row selectline 306. When row select line 306 is active these two switchtransistors are on, that is the switches are “closed”, and there is arelatively low impedance connection between lines 315, 317 and 308. Whenrow select line 306 is inactive transistors 320 and 322 are switched offcapacitor 314 and the gate of transistor 310 are effectively isolated,and any voltage set on capacitor 314 is memorised.

A photodiode 316 is coupled between GND line 302 and line 317 so that itis reverse biased. The photodiode is physically arranged with respect tothe OLED display element 312 such that an optical feedback path 318exists between OLED 312 and photodiode 316. In other words, OLED 312illuminates photodiode 316 and this allows an illumination-dependentcurrent to flow in a reverse direction through photodiode 316, that isfrom GND line 302 towards V_(ss). As the skilled person will understand,broadly speaking each photon generates an electron within photodiode 316which can contribute to a photocurrent.

Column data line 308 is coupled, at the end of a column, to programmablereference current generator 324. This attempts to cause a referencecurrent, which will be referred to as I_(col), to flow to off-pixelV_(ss) connection 326. Line 317 may be referred to as a current senseline, passing a current I_(sense) and line 315 may be referred to as acontrol line, passing a current I_(error) to set a voltage on capacitor314 to control OLED 312. When row select line 306 is active andtransistors 320 and 322 are on I_(col)=I_(sense)+I_(error) and thus acurrent I_(error) flows either onto or off capacitor 314 until OLED 312illuminates photodiode 316 such that I_(sense)=I_(col). At this pointrow select line 306 can be deactivated, and the voltage required forthis level of brightness is memorised by capacitor 314.

Similarly to FIG. 2 a, as drawn the maximum (most negative) gate voltagedrive for transistor 310 is V_(ss) and to permit a greater (morenegative) drive off-pixel connection 326 may be connected to a drivevoltage V_(drive) more negative than V_(ss).

The time required for the voltage on capacitor 314 to stabilise dependsupon a number of factors, which may be varied in accordance with thedesired device characteristics, and may be a few microseconds. Broadlyspeaking a typical OLED drive current is of the order of 1 μA whilst atypical photocurrent is around 0.1% of this, or of the order of 1 nA (inpart dependent upon the photodiode area). It can therefore be seen thatthe power handling requirements of transistors 320 and 322 arenegligible compared with that of the drive transistor 310, which must berelatively large. To speed up the settling time of the circuit it ispreferable to use a relatively small value for capacitor 314 and arelatively large area photodiode to increase the photocurrent This alsohelps reduce the risk of noise and stability at very low brightnesslevels associated with stray or parasitic capacitance on column dataline 308.

FIGS. 3 b and 3 c show a portion of the circuit of FIG. 3 a illustratingdifferent possible configurations for switching transistorscorresponding to switching transistors 320 and 322 of FIG. 3 a. Thepurpose of transistors 320 and 322 is to couple lines 315, 317 and 308when row select line 306 is active and it will be appreciated that thereare three different ways of connecting three nodes using twocontrollable switches.

In FIG. 3 b a first switching transistor 350 is connected between lines308 and 315 and a second switching transistor 352 is connected betweenlines 315 and 317. Both transistors 350 and 352 are controlled by rowselect line 306. In FIG. 3 c a first switching transistor 360 isconnected between lines 308 and 315 and a second switching transistor362 is connected between lines 308 and 317. Optionally a third switchingtransistor 364 may be connected between lines 315 and 317. The two (orthree) switching transistors are all controlled by row select line 306.

The preferred photosensor is a photodiode which may comprise a PN diodein TFT technology or a PIN diode in crystalline silicon. However otherphotosensitive devices such as photoresistors and photosensitive bipolartransistors and ETs may also be employed, providing they have acharacteristic in which a photocurrent is dependent upon their level ofillumination.

The active matrix pixel circuits as described use PMOS transistors butthe circuits may be inverted and NMOS employed or, alternatively, acombination of PMOS and NMOS transistors or bipolar transistors may beused. The transistors may-comprise thin film transistors (TFTs)fabricated from amorphous or polysilicon on a glass or plastic substrateor conventional CMOS circuitry may be used. Alternatively plastictransistors such as those described in WO 99/54936 may be employed, andhe photodiode may comprise a reverse biased OLED to allow the entirecircuitry to be fabricated from plastic. Although PMOS is preferably forthe amorphous pixel driver transistors, external integrated circuitdrivers fabricated on conventional silicon will generally employ NMOStransistors.

Referring now to FIG. 4, this shows an organic LED active matrix pixeldriver circuit 400 which can be operated in a number of different modes,as described in UK patent application number 0126122.1.

As shown, the pixel driver circuit is provided with a ground (GND) line402, a power or V_(ss) line 404, row select lines 406, 407 and a columndata line 408. A reference current source (or sink) 424, preferably aprogrammable constant current generator, allows a current in column dataline 408 to be adjusted to a desired level to set a pixel brightness. Inother arrangements, however, a programmable voltage generator may beused additionally or alternatively to current generator 424, to allowthe driver circuit to be used in other modes. Row driver circuitry 432controls the first and second row select lines 406 and 407 according tothe operating mode of the pixel driver circuitry.

The pixel driver circuit 400 comprises a driver transistor 410 connectedin series with an organic LED display element 412 between the GND 402and V_(ss) 404 lines. A storage capacitor 414, which may be integratedwith the gate of transistor 410, stores a charge corresponding to amemorised gate voltage to control the drive current through OLED element412.

Control circuitry for the pixel driver comprises two switchingtransistors 420, 422 with separate, independently controllable gateconnections coupled to first and second select lines 406 and 407respectively. A photodiode 416 is coupled to a node 417 betweentransistors 420 and 422. Transistor 420 provides a switched connectionof node 417 to column data line 408. Transistor 422 provides a switchedconnection of node 417 to a node 415 to which is connected storagecapacitor 414 and the gate of transistor 410. Again, preferably all thetransistors of the pixel driver are PMOS.

As before a photodiode 416 is coupled between GND line 402 and line 417so that it is reverse biased. The photodiode is physically arranged withrespect to the OLED display element 412 to provide an optical feedbackpath 418, so that an illumination-dependent current flows in a reversedirection through photodiode 416, that is from GND line 402 towardsV_(ss).

When first select line 406 is active transistor 420 is on, that is theswitch is “closed” and there is a relatively low impedance connectionbetween column data line 408 and node 417. When first select line 406 isinactive transistor 420 is switched off and photodiode 416 iseffectively isolated from column data line 408. When second select line407 is active transistor 422 is switched on and nodes 415 and 417 arecoupled; when second select line 407 is inactive transistor 422 isswitched off and node 415 is effectively isolated from node 417.

It can be seen that when both transistors 420 and 422 are switchedoffside both the first and second select lines 406 and 407 are inactive)photodiode 416 is effectively isolated from the remainder of the drivercircuitry. Similarly when transistor 422 is off(second select line 407is inactive) and transistor 420 is on (first select line 406 is active)photodiode 416 is effectively connected between ground GND) line 402 andcolumn data line 408. In this way photodiode 416 may be effectivelyisolated from the remainder of the driver circuitry and used as asensor.

The active matrix pixel driver circuitry 400 may be operated in acurrent-controlled mode with optical feedback, in a voltage-controlledmode with optical feedback, and in a voltage-controlled mode withoutoptical feedback. Any or all of these modes may be employed with a lightmeasurement mode to make an ambient light measurement before data iswritten to a pixel, or to input an image after data is written to apixel.

The pixel driver circuit has a first mode of operation which, broadlyspeaking, is a previously described. In this mode first and secondselect lines 406 and 407 are connected together or driven in tandem byrow drivers 432 so that the circuit operates as a current-controlleddriver with optical feedback. As before, the programmable referencecurrent generator 424 attempts to cause a reference current I_(col) toflow to off-pixel V_(ss) connection 426. Again off-pixel connection 426may be connected to a drive voltage V_(drive) more negative than V_(ss)to permit a greater (more negative) drive to the gate of transistor 410.

In this first mode line 417 may be referred to as a current sense line,passing a current I_(sense) and line 415 may be referred to as a controlline, passing a current I_(error) to set a voltage on capacitor 414 tocontrol OLED 412. As before, when first and second (row)select lines 406and 407 are active transistors 420 and 422 are on andI_(col)=I_(sense)+I_(error) and thus the current I_(error) flows eitheronto or off capacitor 414 until OLED 412 illuminates photodiode 416 suchthat I_(sense)=I_(col). At this point the first and second row selectlines 406 and 407 can be deactivated and the voltage required for thislevel of brightness is memorised by capacitor 414.

In a second mode the pixel driver circuitry 400 is voltage controlledand operates in a similar manner to the prior art circuit of FIG. 1 b,that is without optical feedback. As in the first mode of operation, thefirst and second select lines are connected together or driven in tandemby row drivers 432 but instead of column data line 408 being driven by areference current generator 424, line 408 is driven by a voltagereference source, programmable to adjust the pixel brightness. Thevoltage source preferably has a low internal resistance to approximate aconstant voltage source.

In this second mode of operation when the first and second select lines406 and 407 are active capacitor 414 is coupled to column data line 408and is therefore charged to the voltage output by the reference voltagegenerator. The small reverse current through photodiode 416 due toillumination by OLED 412 has a substantially no effect on the voltage online 408 because of the low internal resistance of the voltage source.Once capacitor 414 has been charged to the required voltage transistors420 and 422 are switched off by deasserting the first and second selectlines 406 and 407, so that capacitor 414 does not discharge throughphotodiode 416. In this mode of operation the pair of transistors 420and 422 effectively perform the same function as transistor 162 in thecircuit of FIG. 1 b.

In a third mode of operation the circuit is again driven by aprogrammable reference voltage source but the second select line iscontrolled so that it is always active (and hence so that transistor 422is always on) whilst OLED 412 is on. In this way photodiode 416 isconnected across storage capacitor 414 so that the circuit operates insubstantially the same way as the circuit of FIG. 2 b described above,transistor 420 performing the function of transistor 260 in FIG. 2 b. Ina simple embodiment the second select line 407 may simply be tied to afixed voltage supply to ensure this line is always active. Howevertransistor 422 need only be on long enough to ensure that capacitor 414has enough time to discharge and thus it is still possible in this modeto switch off transistor 422 at times to allow photodiode 416 to beconnected between lines 402 and 408 by transistor 420 and used as asensor.

In an improvement of this mode of operation the programmable referencevoltage source can be arranged to deliver a predetermined charge tocapacitor 414 sine, when photodiode 416 is connected across capacitor414, it is the charge on capacitor 414 which determines the apparentbrightness of OLED 412 rather than the voltage itself Delivering apredetermined charge to capacitor 414, rather than charging thecapacitor to a reference voltage, reduces the effect of non-linearitiesin the charge-voltage characteristic of capacitor.

The pixel driver circuitry 400 may be controlled to provide ameasurement cycle before pixel illumination data is written to thecircuit to set the brightness of OLED 412. In the above described modesit will be recognised that the first select line 406 in effect operatesas a row select line whilst the second select line 407 operates as acombined mode and row select line. Thus, for example, in order toperform a (write black)—(measure)—(write level) cycle for a selected rowthe first select line 406 is held active whilst the second select line407 is toggled from active during a write cycle to inactive ordeasserted during a measure cycle.

FIG. 5 shows (not to scale) two alternative physical structures for OLEDpixel driver circuits incorporating optical feedback FIG. 5 a shows abottom-emitting structure 500 and FIG. 5 b shows a top-emitter 550.

In FIG. 5 a an OLED structure 506 is deposited side-by-side withpolysilicon pixel driver circuitry 504 on a glass substrate 502. Thedriver circuitry 504 incorporates a photodiode 508 to one side of theOLED structure 506. Light 510 is emitted through the bottom (anode) ofthe substrate.

FIG. 5 b shows a cross section through an alternative structure 550which emits light 560 from its top (cathode) surface. A glass substrate552 supports a first layer 554 comprising the driver circuitry andincluding a photodiode 558. An OLED pixel structure 556 is thendeposited over the driver circuitry 554. A passivation or stop layer maybe included between layers 554 and 556. Where the pixel driver circuitryis fabricated using (crystalline) silicon rather than polysilicon oramorphous silicon a structure of the type shown in FIG. 5 b is requiredand substrate 552 is a silicon substrate.

In the structures of FIGS. 5 a and 5 b the pixel driver circuitry may befabricated by conventional means. The organic LEDs may be fabricatedusing either ink jet deposition techniques such as those described in EP880303 to deposit polymer-based materials or evaporative depositiontechniques to deposit small molecule materials. Thus, for example,so-called micro-displays with a structure of the type illustrated inFIG. 5 b may be fabricated by ink jet printing OLED materials onto aconventional silicon substrate on which CMOS pixel driver circuitry haspreviously been fabricated.

With all these arrangements, however, it is generally desirable toreduce the power consumption of the active matrix display, and moreparticularly of the combination of the display and its (generallyexternal) driver circuitry. It is flier desirable to reduce the maximumrequired power supply voltage for the display plus driver combination.

According to the present invention there is therefore provided a displaydriver for an electroluminescent display, the display comprising aplurality of electroluminescent display elements each associated with adisplay element driver circuit, each said display element driver circuitincluding a drive transistor having a control connection for driving theassociated display element in accordance with a voltage on the controlconnection, the display driver comprising at least one display elementbrightness controller to provide an output to drive a said controlconnection to control the electroluminescent output from a said displayelement; a voltage sensor to sense the voltage on a said controlconnection; and a power controller for controlling an adjustable powersupply for providing an adjustable voltage to said electroluminescentdisplay to power said drive transistors for driving said displayelements, said power controller being configured to provide a controlsignal to adjust said power supply voltage in response to said sensedvoltage.

Sensing the voltage on a drive transistor control connection allows thestrength of drive to be gauged and thus allows excess power dissipationin a drive transistor to be reduced by adjusting, and preferablyreducing, the power supply accordingly. More particularly where thevoltage on a control connection is less than the maximum available thevoltage on the control connection may be increased thus permitting areduced voltage, power supply for the electroluminescent displayelements and their associated driver transistors. The voltage on a saidcontrol connection will generally be sensed indirectly by sensing thevoltage on a control line of the display, such as a column (or row)control line of an active matrix display. Depending upon the type ofdrive to the display, that is for example whether current or voltagedrive is employed, an adjustment to the power supply voltage may bringabout an automatic adjustment to the voltage on the drive transistorcontrol connection.

In a preferred embodiment the drive transistor comprises a FET (orMOSFET) and the control connection comprises a gate connection of thetransistor. Thus the voltage sensor senses the gate voltage of a drivetransistor, and this may be accomplished by monitoring the voltage on acontrol line connection to the display. Even where the display elementbrightness controller provides a current rather than a voltage drive,sensing the voltage on a (current) control line nonetheless may, ineffect, sense the gate voltage of a drive transistor. Thus the displaydriver may be employed with a conventional, unmodified active matrixdisplay to increase the power efficiency of the display plus drivercombination.

To optimise the efficiency of the display and driver combination it ispreferable to use as small power supply voltage as possible. Therequired power supply voltage will, in part, be determined by thedisplayed image and hence by the data written to the display. Moreparticularly the minimum usable power supply voltage will, in part, bedetermined by the power supply requirements of the brightest illuminateddisplay element, and preferably the power supply voltage is no greaterthan required by this(or these) display element (or elements). Howeverthe minimum usable power supply voltage will also depend upon how hardthe drive transistors may be driven on their control connections and,more particularly by the maximum drive available for the brightestilluminated pixel. It is therefore preferable to adjust the power supplyuntil the control connection or gate voltage increases to the maximumavailable for driving the display and, as previously mentioned, thisgate voltage may be monitored by monitoring a control line of thedisplay. It will be appreciated that, generally speaking, reducing thepower supply voltage will have the effect of increasing the controlconnection voltage since normally there is a mechanism for driving thedisplay to produce a controlled brightness so that when the power supplyvoltage is reduced the control connection voltage is increased tocompensate. This function may be performed by the display elementbrightness controller. An alternative way of picturing this mechanism isto consider it as control of the control connection or gate voltage topermit a reduction in the power supply voltage, although in practisethis is less convenient to implement as a knowledge of the drivetransistor characteristics may be required.

It will be appreciated that the brightness of a display element could bemonitored, for example using a photodiode, to allow adjustment of thepower supply voltage until the brightest illuminated element starts toget dimmer but it has been recognised that brightness information can,in effect, be derived more simply by monitoring a drive level, moreparticularly a drive transistor control connection voltage. It has alsobeen recognised that this voltage may, in turn, be monitored bymonitoring a brightness control connection to the display such as acurrent or voltage-controlled brightness setting line or connection.

In a preferred embodiment the display is an active matrix display with aplurality of row and column connections, for example, pixel select linesbeing connected to the row connection and pixel brightness control linesbeing connected to the column connections. The voltage sensor may then,for example, sense the voltage on a brightness control or columnconnection.

In one embodiment the brightness controller comprises a substantiallyconstant current generator, preferably adjustable to provide adjustabledisplay element brightness. The constant current generator may compriseeither a current source or a current sink. The voltage on a controlconnection of the display may then be substantially determined by avoltage level (input or output) of the constant current generator, whichdepends upon a current supplied by the generator. The power controllermay then be configured to reduce the power supply voltage when thesensed voltage on a control connection is less in absolute terms (thatis ignoring polarity) than a threshold voltage such as a maximumavailable voltage for driving the display. The sensed voltage forcomparison with the threshold voltage preferably comprises a voltagesensed from a display element having a maximum brightness relative toothers of the display elements at a given time, that is the brightestilluminated display element. It will be recognised that there may bemore than one such pixel and that where the display is, for example,partitioned into sections with different drivers the maximum brightnessof a display element in the appropriate partition for the driver may beemployed.

In another embodiment the display element driver circuits are similar tothe circuit described above with reference to FIG. 2 b, that isvoltage-controlled with a photo diode to provide optical feedback sothat the voltage on the drive transistor control connection decays withtime. In this embodiment the power controller may be configured toreduce the power supply voltage when the control connection voltage ofthe brightest illuminated display element has reduced to less than afirst threshold value after a predetermined interval such as a lineinterval, frame interval or other cycle interval. The first thresholdvalue may comprise, for example, a gate-source threshold voltage V_(T)of a FET or a base emitter voltage V_(be) of a bipolar transistor, orsome other threshold value such as 0 volts. Broadly speaking the firstthreshold value is preferably selected to be substantially equal to aminimum control connection voltage required for the drive transistor toturn on. Preferably the power controller is flier configured to increasethe power supply voltage when the control connection voltage has notdecayed to less than a second threshold value, preferably equal to thefirst threshold value, after the predetermined interval.

Embodiments of the display driver may include the adjustable powersupply.

In another aspect the invention provides a power controller for adisplay driver for an electroluminescent display, the display comprisinga plurality of electroluminescent display elements each associated witha display element driver circuit, each said display element drivercircuit including a drive transistor having a control connection fordriving the associated display element in accordance with a voltage onthe control connection, the power controller comprising a memory storingprocessor control code; a processor coupled to the memory for executingsaid processor control code; a sensed voltage input for sensing avoltage on a said control connection; and a control signal output forcontrolling an adjustable power supply for providing an adjustablevoltage to said electroluminescent display to power said drivetransistors for driving said display elements; said processor controlcode comprising instructions for controlling the processor to read saidsensed voltage input and to output a control signal to adjust said powersupply in response to said sensed voltage.

The invention also provides a carrier carrying the above-describedprocessor-control code the carrier may comprise any conventional datacarrier or storage medium such as a hard or floppy disk, ROM, or CD-ROMor an optical or electrical signal carrier.

In another related aspect the invention provides a method of operatingan active matrix electroluminescent display, the display comprising aplurality of pixels each with an associated pixel driver, the displayhaving a power supply and plurality of control lines for setting thebrightness of each pixel, the method comprising setting the brightnesspixels of the display using said control lines; monitoring control linesof the display, and reducing said power supply responsive to saidmonitoring.

The control lines may comprise, for example, column (or row) electrodelines of the display, although the skilled person will recognise thatthe active matrix display need not have pixels in a regular gridpattern. The display may be a colour display and the pixels may be ofdifferent colours or the pixels may all be of substantially the samecolour, albeit preferably of variable brightness rather than merely onor off. The pixel brightness setting and control line monitoring may becombined.

The display pixels may include either a bipolar or FET (or MOSFET)driver transistor connected in series with an electroluminescent displayelement. The monitoring may thus monitor a control voltage of a pixeldrive transistor, such as a base or gate voltage.

With a voltage-driven pixel driver the monitoring may determine whetherthe drive transistor control voltage is sufficient, or whether the powersupply voltage is sufficient, by determining whether the brightest pixelis bright enough. This may be achieved by monitoring the control voltageof the drive transistor of the brightest illuminated pixel.Alternatively with a current drive in which, broadly speaking, the levelof a substantially constant current generator sets the brightness of apixel, the drive transistor control voltage may be monitored todetermine whether or not the drive transistor could be driven harder,thus permitting the power supply voltage to be reduced. The monitoringmay therefore comprise determining a maximum pixel brightness of thepixels which are illuminated (rather than, for example, a maximumpossible pixel brightness) and the power supply may then be reduced tosubstantially no more than required by that maximum pixel brightness.Alternatively the power supply may be controlled so that it does notreduce the power supply voltage to less than required for the maximumrequired pixel brightness.

The minimum required power supply voltage depends upon the controlvoltage of the drive transistor for the brightest illuminated pixel. Thepower supply voltage may be set to the minimum required by reducing thepower supply voltage until the control voltage of the drive transistorincreases to the maximum available control voltage, that is the maximumcontrol voltage which a display driver can provide to the display giventhe available power supply to the display driver. Thus the reducing maycomprise reducing the power supply until the control voltagesubstantially reaches a maximum available control voltage, for instancea maximum voltage available at a control line of the display at thepoint of monitoring.

Where a voltage-driven display with optical feedback is employed suchthat the control voltage decays over time, the monitoring preferablymonitors the decayed voltage, for example after a predetermined timesuch as a frame interval where the voltage decays over a frame interval.The power supply voltage may be reduced if the control voltage,preferably of the brightest illuminated pixel, has decayed to less thana threshold voltage, and may otherwise be increased. In other words ifthe decayed voltage indicates that the pixel is sufficiently brightlyilluminated the power supply voltage may be reduced until it is justsufficient (or just insufficient). As previously mentioned, thethreshold voltage may comprise, for example, a threshold voltage of aFET driver transistor or a base emitter voltage of a bipolar drivertransistor.

The invention also provides an active matrix display driver configuredto operate in accordance with the above-described method. Thus thedisplay driver may incorporate means for setting the brightness ofpixels of the display, means for monitoring the control lines of thedisplay, and means for reducing the power supply responsive to themonitoring.

In the above-described aspects of the invention the electroluminescentdisplay is preferably an organic light emitting diode (OLED)-baseddisplay, such as a small molecule or polymer OLED-based display.

In all the above aspects of the invention the electro-optic orelectroluminescent display element preferably comprises an organic lightemitting diode.

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIGS. 1 a and 1 b show, respectively, a basic organic LED structure, anda typical voltage-controlled OLED driver circuit;

FIGS. 2 a and 2 b show, respectively, a current-controlled OLED drivercircuit, and a voltage-controlled OLED driver circuit with opticalfeedback according to the prior art;

FIGS. 3 a to 3 c show, respectively, a current-controlled OLED drivercircuit with optical feedback, a first alternative switchingarrangement, and a second alternative switching arrangement;

FIG. 4 shows a multimode organic LED driver circuit with opticalfeedback;

FIGS. 5 a and 5 b show vertical cross sections through device structuresof OLED display elements with driver circuits incorporating opticalfeedback;

FIGS. 6 a to 6 c show, respectively, drain characteristics of an activematrix FET driver transistor, a graph of gate drive voltage againstpower supply voltage for constant drive current for an active matrix FETdriver transistor, and a simplified active matrix pixel driver circuit;

FIGS. 7 a and 7 b show active matrix pixel brightness control-circuits;

FIG. 8 shows an active matrix display driver according to an embodimentof the present invention;

FIGS. 9 a and 9 b show, respectively, flow diagrams for power supplyvoltage control procedures for current- and voltage-controlled activematrix pixel driver circuits; and

FIG. 10 shows a circuit diagram of a maximum voltage detector and thedisplay of the active matrix display driver of FIG. 8.

Referring now to FIG. 6 a, this shows drain characteristics 600 for aFET driver transistor of an active matrix pixel driver circuit, such astransistors 212 and 256 of FIGS. 2 a and 2 b, transistor 310 of FIG. 3 aand transistor 401 of FIG. 4. More particularly a set of curves 602,604, 606, 608 is shown each illustrating the variation of drain currentof the FET with drain-source voltage for a particular gate-sourcevoltage. After an initial non-linear portion the curves becomesubstantially flat, and the FET operates in the so-called saturationregion. With increasing gate source voltage the saturation drain currentincreases; below a threshold gate-source voltage V_(T) the drain currentis substantially 0. Typical values of V_(T) are between 1V and 6V.Broadly speaking the FET acts as a voltage controlled current limiter.

FIG. 6 c shows a drive portion 640 of a typical active matrix pixeldriver circuit. A PMOS driver FET 642 is connected in series with anorganic light emitting diode 644 between a ground line 648 and anegative power line V_(ss) 646. FIG. 6 b relates to the circuit of FIG.6 c and shows a graph 620 of gate-source voltage against V_(ss), curve622 illustrating the variation of V_(gs) with V_(ss) for a constantdrain current, that is a constant current through OLED 644. Curve 622comprises a substantially flat portion 624 corresponding to the flatportions of curves 602-608 and a non-linear portion 626. Dashed lines628 and 630 correspond to a maximum available V_(gs).

It will be appreciated from the circuit of FIG. 6 c that, for a givenOLED drive current, the greater V_(ss) the greater the excess (waste)power dissipation in driver transistor 642. It is therefore preferableto reduce V_(ss) as much as possible to reduce this excess dissipatedpower. However it can be appreciated from graph 620 that there is alimit, as indicated by dashed line 630, below which V_(ss) may not bereduced, this limit being determined by the maximum available V_(gs) andthe required OLED drive voltage.

Still referring to FIG. 6 b, as V_(ss) decreases initially V_(gs)changes little and, broadly speaking the operating point of drivertransistor 642 moves along the flat portion of one of curves 602, 608shown in FIG. 6 a. However as V_(ss) continues to decrease V_(gs) mustincrease to maintain a constant I_(d) and hence a constant drive currentthrough OLED 644. The driver circuit operates with optimum efficiencywhen V_(ss) is no greater than necessary, in other words when the supplyvoltage is not substantially greater than that needed to provide adesired OLED drive current when driver transistor 642 is driven with themaximum available drive voltage. The greater V_(ss) the greater I_(d)and hence the greater the OLED drive current although it will berecognised that there will come a point at which FET 642 no longerlimits the drive current through OLED 64, instead the internalresistance of the OLED and other factors dominating to limit the current

Referring next to FIG. 7 a, this shows a conceptual circuit diagram of abrightness control circuit 700 for the active matrix pixel driver 640 ofFIG. 6 c. A drive control circuit 702 is provided either for each pixelor for a column for row) of the active matrix display. The drive controlcircuit 702 has a brightness control input 704 and a drive controloutput 708 driving the gate of transistor 642 with a voltage V_(g). Thisgate voltage may be sensed by means of a connection 710 to drive controloutput 708; in a practical circuit connection 710 may be indirect, forexample via one or more switching transistors. Drive control circuit 702also has a drive sense input 706, for example to sense the drive currentthrough OLED 644 either directly or indirectly, for example by sensingcurrent through a photodiode optically coupled through OLED 644. Sensingarrangements have been previously been described with reference to FIGS.2 a, 3 and 4. The connection to drive sense input 706 is shown as adashed line since although shown conceptually as tapping a point betweentransistor 642 and OLED 644 in practice the sensing arrangementgenerally includes intervening components, or may not comprise thephysical connection shown.

FIG. 7 b shows a more specific conceptual circuit 720 based on thearrangement of FIG. 7 a In FIG. 7 b the function of drive controlcircuit 702 is performed by a current comparator 712, drive sense input706 is a current drive sense input, and brightness control line 704controls an adjustable constant current generator 714. Comparator 712compares the sensed drive current with the constant current from current(source or sink) generator 714 and provides a gate voltage output 708,for example to maintain the current sensed on input 706 substantiallyequal to the current set by constant current generator 714. In practicethe current to voltage conversion may be implemented by a capacitor. Asbefore a comparator 712 may be provided for each pixel or for a set ofpixels, for example for each column of the display.

FIG. 8 shows a block diagram 800 of a display driver for an activematrix display 802, configured to control V_(ss) in accordance with theavailable active matrix pixel drive voltage to increase the powerefficiency of the display plus driver combination.

In FIG. 8 the active matrix display 802 has a plurality of rowelectrodes 804 a-e and a plurality of column electrodes 808 a-e eachconnecting to internal respective row and column lines 806, 810 ofwhich, for clarity, only two are shown. Power (V_(ss)) 812 and ground818 connections are also provided, again connected to respectiveinternal conducting traces 814 and 816 to provide power to the pixels ofthe display. For clarity a single pixel 820 is illustrated, connected asshown to V_(ss), ground, row, and column lines 814, 816, 806, and 810.It will be recognised that in practice a plurality of such pixels isprovided generally, but not necessarily, arranged in a rectangular gridand addressed by row and column electrodes 804, 808. The active matrixpixel 820 may comprise any conventional active matrix pixel drivercircuit, such as the previously described circuits pixel driver circuits200, 250, 300 and 400.

In operation each row of active matrix display 802 is selected in turnby appropriately driving row electrodes 804 and, for each row, thebrightness of each pixel in a row is set by driving, preferablysimultaneously, column electrodes 808 with brightness data. Thisbrightness data as described above, may comprise either a current or avoltage. Once the brightnesses of the pixels in one row have been setthe next row may be selected and the process repeated, the active matrixpixels including a memory element, generally a capacitor, to keep therow illuminated even when not selected. Once data has been written tothe entire display, the display only needs to be updated with changes tothe brightness of pixels.

Power to the display is provided by a battery 824 and a power supplyunit 822 to provide a regulated V_(ss) output 828. Power supply 822 hasa voltage control input 826 to control the voltage on output 828.Preferably power supply 822 is a switch mode power supply with rapidcontrol of the output voltage 828, typically on a microsecond time scalewhere the power supply operates at a switching frequency 1 MHz orgreater. Use of a switch mode power supply also facilitates use of a lowbattery voltage which can be stepped up to the required V_(ss) level,thus assisting compatibility with, for example low voltage consumerelectronic devices.

The row select electrodes 804 are driven by row select drivers 830 inaccordance with a control input 832. Likewise the column electrodes 808are driven by column data drivers 834 in response to a data input 836.In the illustrated embodiment each column electrode is driven by anadjustable constant current generator 840, in turn controlled by adigital-to-analogue converter 838 coupled to input 836. For clarity onlyone such constant current generator is shown.

The constant current generator 840 has a current output 844 to source orsink a substantially constant current The constant current generator 840is connected to a power supply drive V_(drive) 842, which may be equalto (and connected to) V_(ss) but which is preferably greater than V_(ss)(in this example, more negative than V_(ss)) to allow active matrixpixel 820 to be driven harder than V_(ss).

As the skilled person will appreciate, constant current generator 840 ineffect adjusts the voltage on output 844 in order to attempt to maintaina substantially constant current in line 844. Current generator 840 hasa limit to the voltage it can provide which is termed (output voltage)compliance limit. The maximum constant current which can be supplied inline 844 is determined by the level of V_(drive) 842 and the complianceof the constant current generator. Any constant current generator may beemployed, but a particularly advantageous form of constant currentgenerator may be constructed using a bipolar transistor with its emitterand collector terminals directly connected to column line 844 and supplyvoltage V_(drive) 842. This bipolar transistor may be incorporated intoa current mirror, the output current being programmed or controlled by,for example, resistors switched using MOSFETs. Similar techniques aredescribed in the applicant's co-pending UK patent application no.0206062.2.

The voltage for V_(drive) may be provided, for example, by a separateoutput from power supply unit 822.

The embodiment of the display driver illustrated in FIG. 8 shows acurrent-controlled active matrix display in which a column electrodecurrent to set a pixel brightness. It will be appreciated that avoltage-controlled active matrix display, in which the brightness of apixel is set by the voltage on a column line, could also be employed byusing voltage rather than current drivers for column data drivers 834.

The control input 832 of row select drivers 830 and the data input 836of column data drivers 834 are both driven by display drive logiccircuitry 846 which may, in some embodiments, comprise a microprocessor.The display drive logic 846 is clocked by a clock 848 and, in theillustrated embodiment, has access to a frame store 850. Pixelbrightness and/or colour data for display on display 802 is written todisplay drive logic 846 and/or frame store 850 by means of data bus 852.

The display drive logic has a sense input 856 driven from the output ofan analogue-to-digital converter 854. Analogue-to-digital converter 854is used to monitor the voltage on each of column electrodes 808 a-e thatis, for example, the voltage on line 844. To monitor these voltages aplurality of analogue-to-digital converters may be employed or one ormore A/D converters may be time multiplexed to monitor the columnelectrode voltages. The voltages on the column electrodes correspond tothe gate voltages of the pixel driver transistors in a selected row, aswill be explained below for the specific examples of the previouslydescribed pixel driver circuits. Although not explicitly shown in FIG. 8it is desirable, but not essential, also to measure the supply voltageV_(drive) 842, for example for compliance determination. This may bedone by using analogue-to-digital converter 854, either by using aseparate input on the converter or by time multiplexing the converter,or a separate analogue-to-digital converter may be employed to provide aV_(drive) sense signal to display drive logic 846.

In FIG. 2 a when a row is selected transistors 220 and 222 are turned onand thus the column data line 210 is effectively connected to the gateof driver transistor 212. In FIG. 3 a when row select line 306 is activetransistors 320 and 322 are turned on and the gate of driver transistor310 is effectively connected to column data line 308 and thus thevoltage on column data line corresponds to the gate voltage of drivertransistor 310. In a similar way in FIG. 3 b transistor 350 connectscolumn line 308 to driver transistor control line 315, and in FIG. 3 ctransistor 360 connects column line 308 to driver transistor controlline 315. In FIG. 4 column data line 408 is connected to the gate ofdriver transistor 410 when transistors 420 and 422 are on. It cantherefore be appreciated that although the aforementioned circuitsemploy a current to set the pixel brightness, the current in effectdetermines a gate voltage drive level to provide the required brightnessand this gate voltage drive level appears on the relevant column dataline. In the context of FIG. 8 it can be seen that this gate drivevoltage will appear on the current output line 844 of constant currentgenerator 40. It will be appreciated that this is the case whether, asin circuit FIG. 2 a, the constant current generator sets the current inthe driver transistor directly or whether, as for example in FIG. 3 a,the constant current generator sets a current in a photodiode, thedriver transistor being driven such that the OLED brightness is thatrequired by the photodiode current set by the constant currentgenerator.

In the arrangement of FIG. 2 b when row conductor 262 is activetransistor 260 is on and column conductor 264 is connected to the gateof the driver transistor 256. Thus, again, the voltage on the columnconductor 264 corresponds to that on the gate of the driver transistor256, although in the case of FIG. 2 b. It is the voltage on conductor264 which determines the brightness of OLED 254, as described above.

Referring again to FIG. 8, the display drive logic 846 includes a gatevoltage sense unit 858 and a power controller 860. One or both of thesense unit and power controller may be implemented as processor controlcode where the display drive logic 846 includes a processor. The gatevoltage sense unit 858 reads a voltage on sense input 856 and the powercontroller 860 outputs a voltage control signal to input 826 of powersupply unit 822 to control power supply voltage V_(ss) in response tothe sensed input voltage. The operation of the power controller isdescribed in more detail below with reference to FIGS. 9 a and 9 b forcurrent- and voltage-controlled active matrix displays respectively.

FIG. 9 a shows a flow diagram of a procedure which may be implemented bypower controller 860 in embodiments of a display driver for driving acurrent-controlled active matrix display. Broadly speaking the powercontroller 860, in conjunction with the gate voltage sense unit 858 andanalogue-to-digital converter 854 scans all the pixels of display 802 toidentify the brightest illuminated pixel, that is the pixel with themaximum drive transistor gate voltage, and then controls the powersupply to reduce V_(ss) until the maximum gate voltage is substantiallyequal to the maximum voltage available given the level of V_(drive) 842and the compliance of constant current generator 840.

Referring to the flow chart, step S900 the power controller 860 uses thegate voltage sensor 858 to read the gate voltage V_(g) for all thepixels by reading the voltage on column electrodes 808 a-e as each rowof the display in turn is selected. The power controller ten, at stepS902, identifies the maximum V_(g) value of those read which, in effect,identifies the drive for the brightest pixel or pixels. In alternativeembodiments the brightest pixel or pixels may be determined in someother way, for example by interrogating the data in frame store 850 orby tracking the data written to the display using bus 852.

At step S904 the power controller determines whether or not the maximumV_(g) is less than the maximum available V_(g), that is in the circuitof FIG. 8 for example the maximum voltage which could be provided on acolumn drive line such as line 844. If V_(g) is not less than themaximum available there is no scope to educe the power supply voltagewithout reducing the brightness of the brightest illuminated pixel. Morespecifically, however, if V_(g) is not less than the maximum availabledrive voltage the power supply voltage V_(ss) is insufficient and istherefore increased, at step S910. The procedure then loops back to stepS900 to rescan the display so that changes in pixel brightness may bedetected. If desired the V_(g) thresholds for increasing and reducingV_(ss) may be different to provide a degree of hysteresis in the controlof V_(ss), for example making the threshold for reducing V_(ss) higherthan that for increasing V_(ss).

If, at step S904, it is determined that the drive voltage to the displayis less than the maximum available drive voltage the power controller,at step S908, outputs a control signal to switch mode power supply unit822 to reduce the power supply V_(ss) on line 828 to display 802. Theprocedure then again loops back to step S900 to re-check which pixel ismost strongly driven and to recheck whether there is any further scopefor reducing V_(ss). The reduction in V_(ss) at step S908 may be smallso that V_(ss) changes only gradually, which may be appropriate wherethe brightest pixel is, on average, not at maximum illumination or wherethe display is occasionally briefly black (that is non-illuminated).Alternatively the reduction in V_(ss) may be large where, for example, arapid response is preferred.

As V_(ss) is reduced the constant current drive, that is the constantcurrent generator 840 in the arrangement of FIG. 8, automaticallyincreases the drive voltage to the display in order to attempt to drivethe current required by the desired pixel brightness on to the relevantdisplay control line. To read the drive voltage from the pixel or pixelswith the maximum V_(g) the appropriate row of the display may beselected using row select drivers 830 and the voltage read usinganalogue-to-digital converter 854 whilst driving at least the monitoredpixel (and, if necessary, all the pixels of the row to prevent loss ofdata) with its specified current drive using column data drivers 834.

FIG. 9 b shows a flow chart for a similar procedure in which the activematrix display 802 is voltage driven, for example using pixel drivercircuits similar to those shown in FIG. 2 b. In FIG. 9 b, as with FIG. 9a, the procedure initially, at step S920, reads the voltage drive forthe pixels of the display and identifies the pixel with the maximvoltage drive. As described above, in the circuit of FIG. 2 b the gatevoltage of transistor 256 gradually decays according to the brightnessof OLED 254. Thus, at step S922, the drive voltage of the pixel with themaximum gate voltage drive is monitored at the end of the relevant decaycycle, typically the end of a fame period. This function may beperformed actively, for example by controlling row select drivers 830,but preferably is performed during the usual frame scanning processrequired by the circuit of FIG. 2 b, for example by implementing aread-before-write data access cycle. Broadly speaking the procedure thenchecks to see whether the gate voltage has decayed sufficiently toswitch off the OLED associated with the brightest) pixel, that is in thecontext of FIG. 2 b, to check whether photodiode 266 has substantiallyfully discharged gate capacitor 258. If the voltage has decayedsufficiently, that is if the gate capacitor is sufficiently discharged,the associated pixel OLED is sufficiently bright and the power supplyvoltage may be reduced, otherwise the power supply voltage may beincreased. Thus V_(ss) is on/off servo controlled around the point ofmaximum efficiency operation for the display plus driver combination.

In more detail, at step S924 the drive voltage of the pixel with thegreatest drive voltage is compared with a threshold voltage. Thisthreshold voltage may be 0V, for example to check whether the gatecapacitor has completely discharged, but is preferably a threshold gatevoltage of the driver transistor as once the drive voltage falls belowthis threshold voltage the driver transistor will be switched off andthe associated OLED non-illuminated. If the drive voltage is less thanthe threshold voltage the power supply voltage V_(ss) is more thanrequired by the maximum brightness pixel and thus, at step S926, V_(ss)is reduced and the procedure loops back to step 920. If the voltage hasnot decayed to the threshold voltage V_(ss) is insufficient for themaximum required pixel. brightness and thus, at step S928, V_(ss) isincreased and again the procedure looks back to step S920 to re-checkall the pixels. If desired a degree of hysteresis may be incorporatedinto the V_(ss) control by making the threshold drive voltages forreducing and increasing V_(ss) different. More particularly thethreshold or reducing V_(ss) may be lower (smaller in absolute terms)than the threshold for increasing V_(ss).

In the procedures of FIG. 9 a and/or FIG. 9 b some or all of steps S908,S910, S926 and S928, in which the power supply voltage V_(ss) to thedisplay is altered, may include an additional step of rewriting data tothe display, in particular rewriting data setting the brightness ofilluminated pixels of the display. The skilled person will recognisethat changing the power supply to the display will have the effect ofchanging the brightness of the pixels to which data has already beenwritten. This does not represent a significant problem in avoltage-controlled display employing pixels such as shown in FIG. 2 bsince such a display, in any case, is refreshed at regular intervals tocompensate for the decay in the stored pixel voltages. However in acurrent-controlled display refresh of the display may only be carriedout at longer intervals or, in some instances, not at all.

A small change in the overall brightness of the display may not bethought to represent a significant problem and whether or not elementsof the display are refreshed may be determined based upon, for example,the magnitude of the changes to V_(ss) and the rapidity with which thedisplayed data is in any case changing. For example where the data ischanging rapidly rewriting the displayed data may not be considerednecessary. Alternatively the entire display may be scanned and rewrittenat intervals although these intervals, need not correspond to the frameintervals conventionally associated with raster scanned or passivematrix displays as the purpose of the refresh is not to prevent flickerbut merely to compensate for small brightness changes.

The procedures described with reference to FIGS. 9 a and 9 b lendthemselves to digital implementation but the control functions may alsobe implemented in analogue circuitry or in a mixture of digital andanalogue circuitry. In particular, FIG. 10 shows a circuit diagram of amaximum voltage detector which may be employed to determine the maximumvalue of V_(g) in step S902 of FIG. 9 a or in step S920 of FIG. 9 b.

In FIG. 10 each column electrode 808 a-e is connected to a reactivediode 1002 a-e to sample the voltage on each column line. The diode ORarrangement outputs on line 1004 the maximum voltage on any one of thecolumn electrode lines less a diode voltage drop. A peak detect circuit1005 comprises a capacitor 1006 to store the voltage on line 1004 and acontrollable switch 1008 which is closed in response to a signal onreset line 1010 to reset the charge on capacitor 1006. The maximumdetected voltage output on line 1004 may be buffered with a high inputimpedance amplifier. The reset line 1010 maybe controlled by displaydrive logic 846 of FIG. 8 and the maximum column voltage output on line1004 may be provided to an analogue digital converter, such as ADC 854of FIG. 8, for digitisation prior to inputting to display drive logic846. In this way the sensing circuitry and ADC 854 may be simplified.

Circuits and methods have been described with reference to their use fordriving organic LEDs but the circuits and methods may also be employedwith other types of active matrix electroluminescent display such asinorganic TFE (Thin Film Electroluminescent) displays, gallium arsenideon silicon displays, porous silicon displays, and the like. The circuitsand methods are not restricted to use with displays with pixel drivercircuits of the types shown but may be employed with any display inwhich a current controls a display characteristic. Similarlyapplications of the invasion are not limited to displays comprising agrid of pixels but may also be used with, for example, segmenteddisplays.

No doubt many other effective alternatives will occur to the skilledperson and it should be understood that the invention is not limited tothe described embodiments.

1. A display driver for an active matrix electroluminescent display, thedisplay comprising a plurality of electroluminescent pixels each pixelcomprising a pixel driver circuit and a display element, each said pixeldriver circuit including a drive field effect transistor having a gateconnection for driving the associated display element in accordance witha voltage on the gate connection to produce a driving current throughsaid display element, the display driver comprising: a display elementbrightness controller configured to provide an output to drive a saidgate connection to control the electroluminescent output from a saidpixel; a voltage sensor to sense a said voltage on said gate connection;and a power controller coupled to said voltage sensor for controlling anadjustable voltage power supply to each of said plurality ofelectroluminescent pixels, said power controller configured to read asensed voltage on each said pixel gate connection within a predeterminedperiod to identify a display element having a maximum brightnessrelative to others of said display elements within said predeterminedperiod, wherein said display element brightness controller and saidpower controller are configured to, respectively and concurrently withina period, increase said voltage on said gate connection of said pixelhaving said identified display element and to reduce said power supplyvoltage, to a point where the voltage of said adjustable voltage powersupply is just sufficient to maintain a current to said identifieddisplay element substantially equal to a predetermined currentcorresponding to a current that is produced in said identified displayelement prior to said increasing of said voltage on said gate connectionand said reducing of said power supply voltage, said increasing and saidreducing in response to a said sensed voltage on said gate connection ofsaid pixel having said identified display element, wherein saidincreasing and said reducing are performed as long as said sensedvoltage on said gate connection is determined to be less than a maximumavailable voltage for outputting from said brightness controller to saiddisplay element and until said voltage on said gate connectionsubstantially reaches said maximum available voltage.
 2. A displaydriver as claimed in claim 1 wherein said voltage sensor is configuredto sense the voltage on a said gate connection by sensing the voltage onan electrode of said display.
 3. A display driver as claimed in claim 1wherein a said pixel includes a photodiode, and wherein a photocurrentthrough said photodiode is determined by a said adjustable constantcurrent to determine a brightness of said pixel.
 4. A display driver asclaimed in claim 1 wherein said power controller is further configuredto increase said power supply voltage when said gate connection voltageof said brightest pixel has not reduced to less than a threshold valueafter a predetermined interval.
 5. A display driver as claimed in claim1 further comprising said adjustable voltage power supply.
 6. A methodof operating an active matrix electroluminescent display, the displaycomprising a plurality of electroluminescent pixels each pixelcomprising an associated pixel driver and a display element, each saidpixel driver including a drive field effect transistor having a gateconnection for driving the associated display element in accordance witha voltage on the gate connection to produce a driving current throughsaid display element, the display having an adjustable voltage powersupply coupled to provide a power supply voltage to each of saidplurality of electroluminescent pixels, and a plurality of control linescorresponding to said gate connections for setting the brightness ofeach pixel, the method comprising: setting the brightness of pixels ofthe display using said control lines to drive said gate connections;monitoring said control lines of the display to sense said voltages onsaid gate connections, and controlling said adjustable voltage powersupply to each of said plurality of electroluminescent pixels by readinga sensed voltage on each said gate connection within a predeterminedperiod to identify a display element having a maximum brightnessrelative to others of said display elements within said predeterminedperiod; and concurrently within a period, reducing said power supplyvoltage and increasing said voltage on said gate connection of saidpixel having said identified display element, responsive to saidmonitoring to a point where a voltage of said adjustable voltage powersupply is just sufficient to maintain a current to said identifieddisplay element substantially equal to a predetermined currentcorresponding to a current that is produced in said identified displayelement prior to said increasing of said voltage on said gate connectionand said reducing of said power supply voltage, said increasing and saidreducing in response to a said sensed voltage on said gate connection ofsaid pixel having said identified display element, wherein saidincreasing and said reducing are performed as long as said sensedvoltage on said gate connection is determined to be less than a maximumavailable voltage for outputting to said display element and until saidvoltage on said gate connection substantially reaches said maximumavailable voltage.
 7. A method as claimed in claim 6 wherein a saidpixel includes a photodiode and wherein a current through saidphotodiode is determined by an adjustable constant current.
 8. An activematrix display driver configured to operate in accordance with themethod of claim
 6. 9. A display driver as claimed in claim 1 whereinsaid electroluminescent display comprises an organic light emittingdiode display.
 10. A method as claimed in claim 6 wherein saidelectroluminescent display comprises an organic light emitting diodedisplay.